1. Technical Field
This invention relates generally to ink jet printing, and more specifically to multi-density printing by ink jet printheads.
2. Background Art
Commonly assigned, co-pending U.S. Pat. No. 5,880,759 filed in the name of K. Silverbrook and corresponding to PCT/US96/04887 filed Apr. 9, 1996, discloses a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with drop size and placement accuracy, attainable printing speeds, power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks. FIG. 1 shows a single microscopic nozzle tip according to the Silverbrook disclosure. Pressurized ink 100 extends from the nozzle, which is formed from silicon dioxide layers 102 with a heater 103 and a nozzle tip 104. The nozzle tip is passivated with silicon nitride. The "Silverbrook" technique provides for low power consumption, high speed, and page-wide printing. In such ink jet printheads, the energy barrier for ejecting an ink droplet is reduced by reducing the surface tension of the ink solution. Referring to FIGS. 2a-2d, the ink solution in an ink reservoir is under a static pressure so that a ink meniscus is bulged outward at a nozzle outlet (FIG. 2a). For each selected nozzle, a voltage pulse is applied to a ring-shaped resistor. The heating of the resistor by the electric pulse reduces the surface tension of the ink solution in the vicinity of the rim of the nozzle. The heated ink solution is pushed outward by the static pressure (FIG. 2b). The interplay between the surface tension reduction by heating and the static pressure begins to dominate (FIG. 2c), and finally ejects the ink droplet to a receiver media (FIG. 2d). The separation of the droplet from the nozzle can be assisted by a static electric field applied that attracts the ink droplet toward the receiving media.
For many digital printing applications, it is most desired to print in more than two density levels. The present invention provides a printhead architecture that is capable of printing multiple density levels (more than 1 bit) per pixel using the Silvebrook printing technique.
Several methods of printing multiple density levels have been disclosed in the prior art. U.S. Pat. No. 4,353,079 disclosed a thermal ink jet recording apparatus in which a single nozzle is capable of printing multiple droplet sizes. Difficulties occur in this technique when more than one droplet is needed to achieve certain density levels. The print head needs either to stop at a pixel location so that all droplets of different size intended for that pixel are printed before moving to the next pixel, or the different droplets intended for each pixel need to be deflected to the same pixel location while the print head is moving relative to the media. The former approach significantly would decrease printing speed, and the latter is extremely difficult to achieve.
U.S. Pat. No. 4,746,935 and U.S. Pat. No. 5,412,410 disclose ink jet printheads that include multiple nozzles of different diameters. The different diameters lead to ink droplets different in volumes, resulting in multiple density levels on the receiver medium. This technique has practical difficulty in achieving a wide enough dynamic range in the nozzle diameters. At high resolution digital printing, it is required that the biggest droplet be small in volume so that a single droplet is compatible with the pixel size. On the other hand, the minimum nozzle diameter is also restricted by the ink fluid dynamics within the nozzle. When the ink is pushed outward in a ejection, the ink fluid needs to overcome a significant resistance caused by the static nozzle front plate and the ink channel surface. This resistive interaction is most active within a decay length of the physical boundary, that depends on the ejection kinetics as well as the properties of the ink and the nozzle. The nozzle diameter is required to be significantly larger than twice the above decay length to allow a free channel for the ink flow. The combination of the two requirements limits the dynamic range of the print density in the prior art technique. Secondly, for the Silverbrook-type ink jet printhead, the limitation on the dynamic range would be even more stringent. The Silverbrook technique uses back pressure to form a bulged meniscus at the nozzle exit. When the nozzle diameter is large, the ink will flow out across the surface of the front plate. In addition, since Silverbrook does not have additional mechanical driving force on selected ink (other than the static back pressure), the ejection speed of the droplet is very strongly dependent on the dragging force from the physical boundaries of the nozzle. The nozzle diameter must be above a value that is higher than the "no-flow" limit as described above so that the speed benefit of page-wide printing is not lost to decreased firing rate per line. Finally, manufacture variabilities in nozzle diameters are relatively larger for smaller nozzles. For Silverbrook printheads, for example, these variabilities affect the meniscus shape of the ink fluid at the nozzle exit, which in turn affect droplet volume and ejection rate.